1. Field of the Invention
The invention relates to a buttonhole sewing machine for forming a cutout through a work fabric by means of a cutting mechanism and forming a zigzag stitch around the periphery of the cutout.
2. Description of Related Art
Referring to FIG. 7, a conventional buttonhole sewing machine for forming an eyelet type buttonhole forms a cutout 2 through a work fabric 1. The cutout 2 consists of an eyelet portion 2a and a straight portion 2b. Hereinafter, this type of cutout 2 is referred to as an eyelet hole 2. After forming eyelet hole 2, the buttonhole sewing machine automatically forms a continuous zigzag stitch 3 around the periphery of the eyelet hole 2 in the order shown by arrows A, B, and C in FIG. 7, that is, firstly on the right side of the straight portion 2b, secondly around the eyelet portion 2a, and finally on the left side of the straight portion 2b.
The buttonhole sewing machine has an arm and a bed. The arm is provided with a needle bar that is moved vertically and swung side to side by a driving mechanism.
The bed is provided with a looper driven in synchronization with the needle bar by a driving mechanism. On the bed, there is provided a feed table on which the work fabric 1 is set. The feed table is moved by a feeding mechanism.
The bed is further provided with a cutting mechanism for forming the eyelet hole 2. The cutting mechanism includes a lower knife and a hammer. The lower knife is fixed at a position spaced apart from the looper and has a cutting blade corresponding to the shape of the eyelet hole 2. The hammer is driven toward the lower knife by an air cylinder.
The buttonhole sewing machine is controlled by a control device including a microcomputer. Under the control of the control device, the buttonhole sewing machine forms the eyelet hole 2, shown in FIG. 7, through the work fabric 1 set on the feed table by means of the cutting mechanism.
Further, as shown in FIG. 8A, the buttonhole sewing machine moves the feed table, on which the work fabric 1 is set, by the feeding mechanism in such a manner that a center of swing of the needle bar moves, relative to the feed table, along a two-dot chain line a (which will be hereinafter referred to as a zero bight line a), and simultaneously drives the needle bar and the looper by the driving mechanisms. As a result, the zigzag stitch 3 is formed around the periphery of the eyelet hole 2 thus forming the eyelet type buttonhole. In forming the zigzag stitch 3 around the semi-circular portion corresponding to an upper half of the eyelet portion 2a, as shown in FIG. 8A, the looper and the needle bar are integrally inverted in a counterclockwise direction as viewed in top plan.
In carrying out the buttonhole sewing as mentioned above, a pre-cutting mode is generally adopted wherein the eyelet hole 2 is formed before formation of the zigzag stitch 3. In the pre-cutting mode, as shown in FIG. 8A, a needle bar swing width or zigzag width L1 is preliminarily set so that left needle location points b of the zigzag stitch 3 formed on the right side of the straight portion 2b of the eyelet hole 2 may substantially coincide with right needle location points b of the zigzag stitch 3 formed on the left side of the straight portion 2b. Further, in the pre-cutting mode, inside needle location points b of the zigzag stitch 3 formed around the periphery of the eyelet portion 2a fall inside the eyelet portion 2a. According to the pre-cutting mode, a peripheral edge of the eyelet hole 2 is covered with the zigzag stitch 3 to thereby obtain a buttonhole having a good appearance.
The pre-cutting mode as mentioned above is adopted for the formation of a buttonhole in the case where the work fabric is formed of a normal fabric material. However, when the work fabric 1 is formed of a fabric material such as a knit which is liable to ravel, it is desirable to adopt an after-cutting mode wherein the eyelet hole 2 is formed after the formation of the zigzag stitch 3.
In the after-cutting mode, the zigzag stitch 3 must be formed so that it is not cut by the cutting mechanism when the eyelet hole 2 is formed. Accordingly, as shown in FIG. 8B, a space must be defined between the left needle location points b of the zigzag stitch 3 formed on the right side of the straight portion 2b of the eyelet hole 2 and the right needle location points b of the zigzag stitch 3 formed on the left side of the straight portion 2b. Further, the inside needle location points b of the zigzag stitch 3 formed in the periphery of the eyelet portion 2a of the eyelet hole 2 must fall outside the eyelet portion 2a. To this end, in the after-cutting mode, the zigzag width is changed into a value L2 smaller than the zigzag stitch L1 in the pre-cutting mode with the feeding operation of the feed table along the zero bight line a being identical to that of the pre-cutting mode. Further, as shown in FIGS. 8A and 8B, the forming position of the eyelet hole 2 for the after-cutting mode deviates a distance .DELTA.Y from that used in the pre-cutting mode. Therefore, it is necessary to adjust the position of the lower knife by the distance .DELTA.Y prior to cutting.
However, in such a buttonhole sewing machine, the driving mechanism for the needle bar is such that rotation of a main shaft is converted into swinging of the needle bar through a needle swinging cam, a cam follower, a driving lever, and a link mechanism. Accordingly, in changing the zigzag width, an operator must operate a zigzag width adjusting mechanism, provided in a portion of the link mechanism, to carry out the fine adjustment of the zigzag width. This fine adjustment is very troublesome and requires much time. The result is a major inconvenience when selecting the pre-cutting mode or the after-cutting mode.